1. Field of the Invention
The invention relates to a display device in a passive matrix structure, which has light emitting diodes such as an organic electroluminescence and a method of displaying an image by using the display device, and specifically relates to a display device having a long product-lifetime and a method of display an image by using the display device.
2. Description of the Related Art
A display device 200 in the related art is shown in FIG. 2. The display device includes an organic panel 1 in which a plurality of light emitting diodes E11˜Emn are formed at each intersection of cathode rays R1˜Rm and anode rays C1˜Cn. Thus, a cathode and an anode of each light emitting diode E11˜Emn are connected to one of the cathode rays R1˜Rm and the anode rays C1˜Cn, respectively. Each light emitting diode E11˜Emn produces luminescence by a drive current, which flows from its anode to its cathode. The display device 200 includes a row driver 2 and a column driver 3, which drive the organic panel 1.
The row driver 2 selectively drives one or some of the cathode rays R1˜Rm periodically with constant interval in the numerical order. The row driver 2 includes a plurality of row switches RS1˜RSm, each of which corresponds to one of the cathode rays R1˜Rm, and the cathode rays R1˜Rm are connected to the ground GND by the operation of the row switches RS1˜RSm in response to a timing signal for scanning.
The column diver 3 drives one or some of the anode rays C1˜Cn of the organic panel 1 in response to data to be displayed (hereinafter called display data). The column diver 3 includes power sources I1˜In supplying a constant drive current, each of which corresponds to one of the anode rays C1˜Cn and column switches CS1˜CSn, each of which turns on/off in response to the display data DT. Thus, when the column switch CS1 turns on and the other column switches CS2˜CSn turns off, only the drive current from the power source I1 is supplied to the anode ray C1. The timing signal for scanning to the row driver 2 and the displayed data to the column driver are provided from a control circuit 4.
The control circuit 4 includes an image data receiving circuit 4a, a memory 4b and a timing controller 4c. The image data receiving circuit 4a receives an image data IN for displaying, and sends the image data to the memory 4b. The memory 4b stores the image data. The timing controller 4c accesses the memory 4b to read out the image data with a constant timing, and send the timing signal to the row driver 2 and the display data to the column driver 3, respectively.
Thus, the display device 200 stores the image data at the memory 4b after the image data IN inputted from an unillustrated external device is inputted at the image data receiving circuit 4a. Based on the image data IN stored in the memory 4b, the timing controller 4c generates the displayed data and the timing signal for scanning, and controls the row switches RS1˜RSm in the row driver 2 and the column switches CS1˜CSn in the column driver 3.
In response to the timing signal from the timing controller 4c, the row driver 2 selectively drives one or more of the cathode rays R1˜Rm periodically with constant interval in the numerical order, as described above. Thus, the electric potential of the selected anode ray(s) is pulled to the ground GND via the selected row switch(es) RS1˜RSm. Further, as descried above, since the timing controller 4c controls the column switches CS1˜CSn in response to the displayed data and in synchronization with the scanning period of the row switches RS1˜RSm, it is possible to supply the drive current to the desired light emitting diodes, selectively.
For example, in the case that the light emitting diodes E11 and E12 produce luminescence, the row driver 2 is scanned in order to turn only the row switch RS1 on so that the electric potential of the anode ray R1 is pulled to the ground GND. Concurrently, the cathode rays C1 and C2 are connected to the power sources 11 and 12 by turning the column switches CS1 and CS2 on. According to this operation, the drive current is supplied to the light emitting diodes E11 and E12, and thus, they produce luminescence.
The luminosity of each the light emitting diodes E11 and E12 is the same because the same drive current is applied to both light emitting diodes E11 and E12. However, it is known to control the luminosity that human feels by utilizing the residual image phenomenon of the human eyes while the same drive current is applied. Changing the time period for supplying the drive current, which is a time period that the column switches CS1 and CS2 turns on during one scanning period, makes this possible. By repeating such scanning operations and driving operations, the image is displayed on the organic EL panel 1.
The Japanese Patent publication Reference JP 2005-107004A discloses a driving device for an organic EL panel in an active matrix structure, which has a low power consumption characteristic by halting an operation of a driving circuit when the display data of a single line indicate all non-luminescence.
The driving device disclosed in JP 2005-107004 A includes a source driver, a gate driver and a control unit for driving the organic EL panel in the active matrix structure. In the driving device, when all displayed data in a single scanning period indicate non-luminescence (ex. Data “0”), the control unit sends an all-zero notice to the source driver. Once the source driver receives the all-zero notice from the control unit, it is compelled to output the black data to each pixel disposed in the organic EL panel. As a result, the operation of the driving circuit is halted. Since the operation of the source driver, which performs high speed operation by a relatively high driving voltage, is temporally halted at the time that all displayed data in a single scanning period indicate non-luminescence, the low power consumption can be expected.
However, in such a display device in the related art, the cathode rays R1˜Rm, which act as scanning lines, are driven by the timing signal whose duty ratio is 1/m (“m” is the number of the cathode rays) in each scanning period. Thus, in order to obtain the displayed luminance Ld required for the display panel, each light emitting diode should produces luminescence with the displayed luminance “Ld×m” when the corresponding cathode ray is driven. For this reason, even the displayed luminance Ld stays constant, it is required to increase the luminescence intensity at each light emitting diode when the number of the scanning lines is increased, that is, when the duty ration 1/m gets smaller. Thus, the drive current to the light emitting diodes also increase in proportion to the increase of the number of the scanning lines.
However, the product-lifetime of the organic EL panel comes under the influence of the amount of the drive current passing though the light emitting diodes. If the amount of the drive current is doubled, the product-lifetime of the organic EL panel may be less than half, such as quarter. Thus, according to the display device in the related art, when the display device displays the image with a contestant luminance, the more the number of the scanning lines increases and the less the duty ration 1/m decreases, the sooner the product-lifetime of the display device is over. The deterioration of the luminance proceeds remarkably with time.